Intrusion detector apparatus - being part of or coupled with security fences, in particular fences comprising optical fibers, or not - are known which comprise taut wires and alarm systems; means are provided for activating the alarm system whenever an intruder cuts the taut cable or attempts to climb over it and therefore causes a displacement in it. In some apparatus disclosed in the prior art, the alarm system is activated whenever light transmission through optical fibers, that are part thereof or associated therewith, is decreased beyond a certain threshold limit, said decrease occurring in response to loads placed on the taut wire.
For instance, U.S. Pat. No. 4,829,286 describes such an apparatus in which a taut wire is mechanically connected to an optical fiber and causes bending of the optical fiber and reduction of the light transmission therethrough whenever the taut wire undergoes displacements due to a load of sufficient magnitude applied to it. However, a device of this kind has several disadvantages. Thus, it is possible by relatively simple mechanical means to neutralize loads on the taut wire, e.g. by applying a suitable traction between two spaced points thereof and then cutting it therebetween. Furthermore, temperature changes beyond a certain limit, winds and other phenomena may cause abnormal displacements in the wire and therefore produce false alarms or, conversely, make the wire less sensitive to load applied to it. Furthermore, the bending of the optical fiber in response to the displacements of the metal taut wire is often not of a magnitude sufficient to provide such a reduction of the light transmission as will provide the alarm, and the cited U.S. patent attempts, by mechanical means, to enhance the bending of the optical fiber to overcome this drawback; and also to avoid false alarms due to slow displacements of the taut wire by providing a viscous material as a component of the mechanism connecting the taut fiber with the optical fiber. However, the structures so provided are complicated and of uncertain efficiency.
It is a purpose of this invention to provide an intrusion alarm system, comprising a tensioned cable and an alarm system, which will provide an alarm in response to the cutting of the cable and to any load or displacement thereof, due to an intrusion attempt, while being insensitive to loads due to other causes, in particular environmental causes, such as temperature changes, wind pressure, etc.
It is another purpose of the invention to provide such a structure which is reactive to localized loads, to cause an alarm, but is not reactive to broadly applied, balanced loads which, by their nature, are not normally due to intrusion attempts.
It is a further purpose of the invention to provide such a structure which requires no displacement enhancing means and is responsive to the loads due to intrusion attempts.
It is a still further purpose of the invention to provide such a structure which is extremely simple and economical and of reliable operation and is free of malfunctions.
Other purposes of the invention will appear as its description proceeds.